1. Field of the Invention
The present invention relates to a head lamp device for a vehicle that includes light emitting diodes.
2. Description of the Related Art
JP-A2004-51014 or its counterpart U.S. Pat. No. 6,870,328 B2, discloses a head lamp device that includes a diode array circuit comprised of plural light emitting diodes. Because the voltage drop of the light emitting diode (hereinafter referred to as LED) is as small as the voltage drop of a common diode in the forward direction, it is easy to adjust its resistance to be connectable to a battery of a certain voltage by connecting a suitable number of LEDs in series. JP-A-10-217851 proposes to detect breakdown of an LED by measuring the impedance thereof. JP-A-2000-98941 discloses an array of series-connected diodes that has a bypass circuit at each junction of the diodes to secure operation of other normal diodes even when one diode breaks down. JP-A-2004-314808 proposes to detect the breakdown of the LED based on a voltage drop of a series-circuit of a current detecting resistor and a series-connected diode.
In such a head lamp device that includes an array of many LEDs, a short circuiting may take place. Because the voltage drop of the LED is very small, decrease in the light intensity of the array as a whole is not so significant even when one or two LEDs are short-circuited. However, the light intensity at a specific area may become lower than a suitable intensity.